Process for reducing the far-end crosstalk between concentric pairs due to tertiary circuits



Oct. 24, 1950 G. FUCHS 2,526,942

PROCESS FOR REDUCING THE FAR-END CROSSTALK BETWEEN CONCENTRIC PAIRS DUETO TERTIARY CIRCUITS Filed March l5, 1947 C5 Sheets-Sheet l oct. 24,195o G C s 2,526,942

. FU H PROCESS FOR REDUCING THE FAR-END CROSSTALK BETWEEN CONCENTRICPAIRS DUE TO TERTIARY CIRCUITS Filed March 15, 1947 5 Sheets-Sheet 2Gafec//o/v Fac/f5,

Oct. 24, 1950 G. FUCHS 2,526,942

PRocEss EoR REDUCING THE EAR-END cRossTALK BETWEEN coNcENTRIc PATRS DUETo TERTIARY CIRCUITS Filed Mann 15, 1947 3 sheets-sheet s' "Sig .I l iN', i z*- ll 5 1 IW l liglll n 115A B- u: a E i E il m E m Patented Oct.24, 10950 UNITED STATES ATENT OFFICE PROCESS FOR REDUCING THE FAR-ENDCROSSTALK BETVQEEN CONCENTRIC PAIRE DUE TO TERTIARY CIRCUITS ApplicationMarch 13, 1947, Serial No. 734,478 In France April 15, 1946 (Cl.lili-78) 9 Claims. 1

It is known that the crosstalk between two concentric pairs isiniluenced by the co-eXistence in the cable of other types of circuitsplaying the part of tertiary circuits, for example symmetrical circuits,such as telephone pairs oi quads or the metallic sheath protecting thecable core.

One also knows that the concentric pairs intended to transmithighfrequencies ci an order of several megacycles per second have smalleramplification sections than those o other circuits so that one isobligated to pro-Vide intermediate repeater stations in which thecurrents transmitted by the concentric pairs are amplified, whereas theother circuits traverse the stations without being cut. Thiscircumstance increases still more the part played by the additionalcrosstalk via the tertiary circuits in the case or long distancecommunications comprising a great number of points of amplification.

In a composite cable comprising several concentric pairs, the Icrosstalkbetween two pairs distant from each other is generally substantiallyconditioned by the indirect crosstalk via the tertiary circuits formedby the cuter tubular conductors or the concentric pairs, Such is thepractical case which arise for instance in a 4 pair concentric compositecable between the diametrically opposite pairs.

It results from what has been exposed hereabove that a compensation ofthe indirect crosstalk between concentric pairs transmitting in the samedirection is highly desirable.

Several methods have already been proposed in view of reducing theprincipal or direct crcsstalk. One oi these known methods consists inopposing the induced currents on a section of the cable by crossing theconductors of a concentric pair in its middle. This method may beembodied for example with aid of a crossing transformer, or again withthe aid of phase shifting networks which make the phase of the currentturn by an angle equal to 1r or still with the aid of a stage of vacuumtubes.

The transposition may besides be made not in the middle of a section ofthe cable, but also at the exit of a line amplifier, the compensationacting then on the entireness of two amplication sections.

The applicant has found that the compensation effected in theseconditions affects but the direct crosstalk, leaving a non-negligiblecomponent of indirect crosstalk which may sometimes render the crossingoperation completely inefficient.

One ci the objects oi the present invention Another object ci theinvention is to reducey both direct and indirect crosstalks by actingsimultaneously on the phase of the induced crosstali; currents and onthe longitudinal transmission characteristics of the outer returnconductors oi the concentric pairs.

According to a further object of the invention the propagation constantsof the concentric pairs and of the tertiary circuits formed by the outerreturn conductors of said concentric pairs are adapted to the totalamplification coeicient of the repeater in a manner to reduce theindirect far-end crosstalk, whereas the direct crosstalk is eliminatedby suitablyY phase shifting the crosstalk currents induced in thedifferent sections of the pair so as to make them compensate oneanother.

With these and otherfobjects in View the present invention mainlyconsists in establishing between the last amplilication section and theoriginating section ci the circuit a total phase shift equal to an oddnumber of 1r, and in adapting the propagation constants of the tertiarycircuits and of the concentric pairs so that the ratio of their sum inrespect to their dilerence should be equal tc the total ampliiicationcoefficient of the repeater.

One embodiment of the present invention will be described hereafter withreference to the accompanying drawing wherein:

Fig. 1d is a diagram illustrating the calculations whereupon theinvention is based.

Fig. l diagrammatically shows the connections between the successivesection of concentric pairs.

Fig. 2 shows the variation of the phase constant of the tertiary circuitas a function of the thickness of an aluminium tape wound on the outerconductor of a pair.

Figs. 3 and 4 show a cable according to the invention in sectional andlongitudinal View respectively.

The method of calculation of the crosstalk between concentric pairs arewell known. u1, i1 being the voltage and the current in the disturbingpair at a point having an abscissae x; u2, iz being the correspondingvoltage and current in the disturbed pair and u', i the correspondingvoltage and current in the tertiary circuit formed by the outerconductors of said pairs, Z0 and g' being the transfer impedance and thepropagation constant of 'the concentric pairs, Zo and g' thecorresronding characteristics of the tertiary circuit, the transferimpedance of the return.

conductor and E the disturbing voltage, this calculation results inintegrating the following system of diierential equations:

This system has been already integrated in many particularly simplecases. I have thoroughly studied the practical case when repeaters areinserted in the circuit and where the transposition method is eventuallyused for the concentric pairs,

Fig. 1d shows a distributing concentric pair I1 provided in the middleof a section having a length I with a line-repeater 2 and with aconnecting device 6 reversing the connections of the conductors beyondthe repeater 2; the pair I1 is connected at one end to a source ofvoltage uo, and closed on its other end on its characteristic impedanceZo. The disturbed pair l2 is provided with repeater 2 and directconnecting device 5, and it is closed on both ends on impedance Zo,whereas the tertiary circuit I is closed on both ends on itscharacteristic impedance Zu. The impedances of the repeaters and of theconnecting devices are supposed to be matched with respect to the lineimpedance.

The disturbing voltage E has not the same value along the whole lengthI: at the left side of the repeater its value is tions, the integratingof the diiferential system shows that the far-end crosstalk voltage u2is proportional to the function Function (p becomes equal to zero if Theattenuation is practically sufficiently high to enable this equality tobe written A=g+g g g which is the condition which is to be satisfied forobtaining the compensation between the crosstalk voltage originatingfrom two amplification sections.

'Ihe calculation above may be generalized rst by taking account of thelead sheath common to both pairs, and second by admitting that thecompensation of the crosstalk voltage is extended to N sections insteadof two sections only.

In this respect the calculations eiected by the applicant have shownthat in the hypothesis that the attenuation of the tertiary circuits ona section of amplification is higher than 20 decibels, hypothesis alwaysveried in practical cases, the

far-end crosstalk potential difference between tw' concentric pairs isproportional to:

where lis the length of a section of amplication N the number ofconnected sections g the propagation constant of the tertiary circuit gthe propagation constant of the concentric pair A the ampliiicationfactor of the repeater (ratio between the output tension and the inputtension) comprising the amplier itself, the distorsion compensatingnetworks, etc.

lcq=+l if the total phase shift between the qm and the iirst sectionsintroduced by the phase shifting devices mentioned above is equal to aneven number of times 1r.

kq=l if this total phase shift is equal to an odd number of times 1r.

The rst term characterizes the direct crosstalk, the two other termscorrespond to the indirect crosstalk.

If the phase shifts are chosen in a manner to compensate the directcrosstalk there remains The indirect crosstalk depends only on the totalphase shift of the last section of amplification.

To render Q as small as possible, one must first of all, and thatconstitutes a rst characteristic of the present invention, make i. e.that the compensation of the indirect crosstalk must be conducted in amanner that the total phase shift introduced by the crossing devicesbetween the last section of amplification and the originating sectionshould be equal to an odd number of 1r,

This condition will be better understood with the aid of the followingexample illustrated by Fig. 1 where a telephone cable comprising twocoaxial pairs I, I and provided with three repeaters 2 betweentransmitter 3 and receiver 4 has been diagrammatically shown. Thereference character 5 denotes direct connections between a section ofthe cable and the repeater and the reference character 6 denotes areversed connection.

Let a crosstalk on 4 amplification sections to be compensated.

A priori, three solutions are conceivable to fulfill the condition (2):

The solutions (la) and (1c) satisfy simultaneously the condition (4).

The indirect crosstalk is then proportional to:

:aF-tv TWA (5) According to a second characteristic of the invention,the relationship .ff-rg A=--- (6 gg is realized in a manner to cancelthe expression of Q.

Among the parameters A, g and g forming the relation (6) theamplification A and the propagation constant g of the coaxial pair maybe considered as given; they are determined as soon as the type of thecoaxial pair has been chosen. In order to practically satisfy tocondition (6), the propagation constant g of the tertiary circuit willbe acted on. The main term of said constant g' is the phase constantapproximately given by the formula a'=w\/LC where w is the angularvelocity 2jr corresponding the considered frequency f, C the capacityand L the self inductance of the tertiary circuit.

If the amplification A has a high value, it is clear that the value ofg' to be chosen must be slightly different from g in order thatdenominator (QN-9) be decreased and the ratio The geometric selfinduction L1 does. not depend on the frequency and the internal selfinduction L2 is a function 0f the surface impedance of the returnconductor of the concentric pairs.

This internal self induction is high when the pairs armouring isconstituted by a steel tape of great permeability; it is low when thearmouring is of non magnetic metal.

The experiments and calculations carried out by the applicant have shownthat a could vary in considerable limits: from 2.2 rad/km. at 60kilocycles per second for a lead sheath type of concentric pairs, to 4.6rad./ km. for a magnetic metal armouring type of concentric pairs. Oneconceives that it is possible to obtain intermediate values of a andconsequently to vary the ratio:

by placing one self between the two limit cases mentioned above. Ifnecessary it is possible of course to obtain greater values of a byusing high permeability alloys.

A first means t0 realize the invention consists in suitably choosing thepermeability of the exterior armouring of the concentric pairs.

For instance, if the propagation constant g is to be increased, a tapeof a high permeability metal instead of an ordinary metal tape will beused.

thickness of the tape in a manner to obtain an internal self inductionL2 corresponding to the One has represented in Fig. 2, by way ofexample, the variation of a in function of the thickness e of thealuminium tape at the frequency frr 60 kilocycles per second.

If one was in need of a very slight thickness, it would be possible toreplace the tape by metallized paper.

Other preconized means consist in utilizing non magnetic tape coiled inform of an open helix over the steel armouring. It is obvious that theeffective permeability of this entireness will place itself between thepermeability equal to the unity of the non magnetic tape and the highpermeability of the magnetic armouring.

The desired value of a is obtained by suitable adjustment of the pitchand of the width and thickness of the tape.

The processes set forth hereabove are only given as indications and haveby no means a limiting character.

Other means may be brought into action to attain the desired object, i.e. to realize the equality:

Azgfrg By way of example one will describe a particular embodiment ofthe invention, as applied to a composite cable which is shown in sectionin Fig. 3. This cable comprises coaxial pairs 1, 1', l, 1 control andservice circuits 8, 8', 8, 8 surrounded by a circular layer of telephonequads 9. The cable is provided with a lead protection I0 and isarmoured. f

The purpose is to compensate the indirect crosstalk between concentricpairs under steel tape armouring, the higher frequency of transmissionbeing 2.8 mc. If one puts to 52 decibels the maximum attenuation on asection of amplication, the attenuation at 60 kc. is of 7.65 decibels.The corresponding coefiicient of amplification is 4422.4, hence onededucts the ratio The phase constant at 60 kc. of the concentric pairbeing a=1.4 rad/km., the phase constant a of the tertiary circuit isa=1.4, 24:34.

Fig. 2 shows that thin value of a may be obtained by using very thinaluminium tape having a thickness slightly below 0.01 mm. and which iswound in such a manner that the successive turns adjoin one another. Ifit is desired to avoid the use of tapes having such a small thickness,it is possible to obtain the same effect by using a tape having agreater thickness, for instance 0.1 mm. but wound in an open helix. n:being the fraction of the surface of the protecting tape of the outerconductor of the pair which is covered by said aluminium tape IE5, thefraction of said surface which remains uncovered is (1-x).

According to the curve shown in Fig. 2,the phase constant is 2.3 rad/km.for e=0.1 mm. and 4.6 rad/km. for e=0, so that the resultant phaseconstant will be given by the expression Equalizing this expression tothe desired value 3.4 rad/km., x is found equal to 0.5, so that the partof the protecting sheath covered by the aluminium tape is approximatelyequal to the partv remaining uncovered.

Fig. 4 is a view of the embodiment described above wherein the innerconductor Il of the coaxial pair is separated from the outer tubularconductor l2 by means of insulating discs i3. The iron tape I4 is woundin a manner such that the turns of the helix partially overlap oneanother, whereas the aluminum tape l5 is wound in the form of an openhelix so as to cover only one-half of the surface of the magnetic iron'.tape.

What I claim is:

l. In a long distance composite telephone I'cable constituted by severalamplification sections, comprising concentric pairs and sym- :metricalcircuits such as twisted pairs and quads :and provided with intermediaterepeaters, means for reducing the indirect far end crosstalk betweenconcentric pairs via tertiary circuits, which comprises means forestablishing between the last section of amplification and theoriginating section of the circuit a total phase shift equal to an oddnumber of 1r and means for adapting the propagation constants of theconcentric pairs to that of the tertiary circuit so that the ratio oftheir sum to their difference be equal to the total amplificationcoefficient of the repeater, means whereby this condition is fuliilledby acting on the internal self induction value of the return conductorsof the concentric pairs.

2. In a long distance composite telephone cable constituted by severalamplification sections, comprising concentric pairs and symmetricalcircuits such as twisted pairs and quads and provided with intermediaterepeaters, means for reducing the indirect far'end crosstalk betweenconcentric pairs via tertiary circuits, which comprises means forestablishing between the last section of amplification and theoriginating section of the circuit a total phase shift equal to an oddnumber of 1r and means for adapting the propagation constants of theconcentric pairs to that of the tertiary circuit so that the ratio oftheir sum to their difference be equal to the total amplificationcoefficient of the repeater, means including an outer sheath, saidsheath consisting of a magnetic metallic tape of suitable permeability,whereby the foregoing condition is fulfilled by said sheath acting onthe internal self induction value of the return conductors of theconcentric pairs.

3. In a long distance composite telephone cable constituted by severalampliiication sections, comprising concentric pairs and symmetricalcircuits such as twisted pairs and quads and provided with intermediaterepeaters, means for reducing the indirect far end crosstalk betweenconcentric pairs via tertiary circuits, which comprises means forestablishing between the last section of amplification and theoriginating section of the circuit a total phase shift equal to an oddnumber of w and means for adapting the propagation constants of theconcentric pairs to that of the tertiary circuit so that the ratio oftheir sum to their difference be equal to the total amplificationcoeiiicient of the repeater, means including a wrapping of non-magneticmaterial whereby the foregoing condition is fulfilled by said wrappingacting on the internal self induction value of the return conductors ofthe concentric pairs.

4. In a long distance composite telephone .gable constituted by severalamplification sections, comprising concentric pairs and symmetricalcircuits such as twisted pairs and quads and provided with intermediaterepeaters, means for reducing the indirect far end crosstalk betweenconcentric pairs via tertiary circuits, which comprises means forestablishing between the last section of ampliiication and theoriginating section of the circuitl a total phase shift equal to an oddnumber of fr and means for adapting the propagation constants of theconcentric pairs to that of the tertiary circuit so that the ratio oftheir sum to their difference be equal to the total amplificationcoefficient of the repeater, means including an aluminum wrappingwhereby this condition is fulfilled by said wrapping acting on theinternal self induction value of the return conductors of the concentricpairs.

5. In a long distance composite telephone cable constituted by severalamplication sections, comprising concentric pairs and symmetricalcircuits such as twisted pairs and quads and provided with intermediaterepeaters, means for reducing the indirect-J far end crosstalk betweenconcentric pairs via tertiary circuits, which comprises means forestablishing between the last section of amplification and theoriginating section of the circuit a total phase shift equal to an oddnumber of 1r and means for adapting the propagation constants of theconcentric pairs to that of the tertiary circuit so that the ratio oftheir sum to their difference be equal to the total amplificationcoefficient of the repeater, means including a metalized paper wrappingwhereby this condition is fulfilled by said wrapping acting on theinternal self induction value of the return conductors of the concentricpairs.

6. In a long distance composite telephone cable constituted by severalamplification sections, comprising concentric pairs and symmetricalcircuits such as twisted pairs and quads and provided with intermediaterepeaters, means for reducing the indirect far end crosstalk betweenconcentric pairs via tertiary circuits, which comprises means forestablishing between the last section of amplification and theoriginating section of the circuit a total phase shift equal to an oddnumber of 1r and means for adapting the propagation constants of theconcentric pairs to that of the tertiary circuit so that the ratio oftheir sum to their difference be equal to the total amplificationcoefficient of the repeater, means including a wrapping formed by a tapewound in an open helix of a particular pitch, thickness and widthwhereby return conductors of the concentric pairs. this condition isfulfilled by said wrapping acting on the internal self induction Valueof the 7. In a long distance composite telephone cable constituted byseveral amplification sections, comprising concentric pairs andsymmetrical circuits such as twisted pairs and quads and provided withintermediate repeaters, means for reducing the indirect far endcrosstalk between concentric pairs Via tertiary circuits, whichcomprises means for establishing between the last section ofamplification and the originating section of the circuit a total phaseshift equal to an odd number of 1r and means for adapting thepropagation constants of the concentric pairs to that of the tertiarycircuit so that the ratio of their sum to their difference be equal tothe total amplification coeiiicient of the repeater, means including awrapping formed by a tape of non-magnetic material wound in an openhelix of a particular pitch, width and thickness whereby this conditionis fulfilled by said tape acting on the internal self induction value ofthe return conductors of the concentric pairs.

8. In a long distance composite telephone cable constituted by Severalamplification sections, comprising concentric pairs and syinmet1 ricalcircuits such as twisted pairs and quads and provided with intermediaterepeaters, means for reducing the indirect far end crosstalk betweenconcentric pairs via tertiary circuits, which comprises means forestablishing between the last section of amplification and theoriginating section of the circuit a total phase shift equal to an oddnumber of 1r and means for adapting the propagation constants o theconcentric pairs to that of the tertiary circuit so that the ratioy oftheir sum to their difference be equal to the total amplificationcoeiicient of the repeater, means including a wrapping formed of a tapehaving a high permeability, said tape being wound in an open helix of aparticular pitch, width and thickness whereby this condition is fullledby said tape acting on the internal self induction Value of the returnconductors of the concentric pairs.

9.1n a long distance composite telephone cable constituted by severalampliiication sections, comprising concentric pairs and symmetricalcircuits such as twisted pairs and quads and provided with intermediaterepeaters, means for reducing the indirect far end crosstali: betweenconcentric pairs via tertiary circuits, which comprises means forestablishing between the last section of ampliiication and theoriginating section of the circuit a total phase shift 10 equal to anOdd number of 1r and means for adapting the propagation constants of theconcentric pairs to that of the tertiary circuit so that the ratio oftheir sum to their difference be equal to the total amplicationcoefficient of the repeater, means whereby the concentric pairs areprovided to this end with a irst magnetic wrapping formed by an irontape overlap wound on the outer return conductor and with a secondnonmagnetic wrapping formed by an aluminum tape would in an open helix,and means whereby the pitch covers only half of the surface of the firstmagnetic wrapping.

GUERCHON FUCHS.

REFERENCES CITED rEhe following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,852,902 Rockewell Apr. 5, 19321,871,906 Nyquist Aug. 16, 1932 1,978,419 Dudley Oct. 30, 1934 2,036,045Harris Mar. 31, 1936 2,111,651 Wentz Mar. 22, 1938 2,119,853r CurtisJune 7, 1938 2,152,706 Mougey Apr. 4, 1939 2,180,731 Dickinson Nov. 21,1939 2,243,851 Booth June 3, 1941 2,245,492 Meyer June 10, 19412,319,744 Mougey May 18, 1943 FOREIGN PATENTS Number Country Date458,225 Great Britain Dec. 15, 1936 653,969 France Mar. 29, 1929 683,391France Mar, 3, 1930

